The Science of Lagering


By Jack Horzempa

The two main processes that occur during lagering is precipitation and flavor maturation.  Some facets of what is occurring during the lagering (cold conditioning) phase is fairly well understood but flavor maturation is still a topic which needs continuing study.

Prior to lagering, how was the lager beer fermented?

What is needed/required from the lagering phase will very much be impacted by what occurred during the fermentation phase. A strong, healthy fermentation will take off some of the pressure during lagering to ‘heal wounds’.

There are a number of fermentation byproducts which occur during primary fermentation which are best addressed during that phase but can be mitigated during lagering if need be.


The compound diacetyl, which can taste like butter, is produced during all fermentations but for a healthy fermentation it is cleaned up by the yeast during the end of fermentation. For lager beers some brewers conduct what is known as a diacetyl rest towards the end of fermentation to encourage the yeast to ‘clean up’ the diacetyl.  This is a step where the temperature of the beer is raised for a few days to help the yeast process this compound. You can learn more here:


If there is some perceptible diacetyl remaining in the beer at the completion of primary fermentation (e.g., beer has reached final gravity) it is possible for the yeast to further process this compound during lagering although the cold conditions are not as well suited for this biochemical process. It is truly best to ensure that no perceptible diacetyl is present before transferring the beer to the lagering vessel.


The chemical compound acetaldehyde, which adds a taste described as green apple, is also produced during fermentation but towards the end of fermentation the yeast processes this compound into alcohol. Just like for diacetyl it is best to conduct a healthy fermentation such that the amount of acetaldehyde at the completion of fermentation is below the flavor threshold. Just like with diacetyl there is a possibility that the yeast during the lagering phase could reduce the acetaldehyde to below flavor threshold but the cold conditions do not favor this occurrence.

Sulfur compounds

Lager yeast strains are different from ale yeast strains in that some lager yeast strains are known to create a number of sulfur compounds. Below is a couple of those sulfur compounds accompanied by aroma/flavor descriptions:

  • Hydrogen Sulfide (H2S) – rotten eggs
  • Sulfur Dioxide (SO2) – struck match

Another sulfur compound sometimes associated with lager beer is Dimethyl Sulfide (DMS) but the principal cause of this compound is from the chemical precursor S-methyl methionine (SMM) prevalent in Pilsner Malt which is converted to DMS during the boiling of wort. A strong, rolling boil followed by rapid cooling of the wort is typically sufficient to mitigate DMS in the resulting beer. When I homebrew a beer which features Pilsner Malt I conduct a vigorous boil for 75 minutes. If you have ever tasted Rolling Rock beer you have tasted DMS which tastes like creamed corn to my palate. This flavor in Rolling Rock is an intended feature and not unpleasant for me but others will think differently here.

Much of the sulfur will be outgassed during the primary fermentation and in the context of Hydrogen Sulfide the rotten egg smell is very noticeable. But there will still be some of the sulfur compounds remaining in the fermented beer. The types and amounts of sulfur compounds in the beer will be fermentation dependent with the specific yeast strain chosen being one variable.

On the topic of Sulfur Dioxide, I have experienced this a number of times with the beer Jever. On some occasions after the initial pour of Jever into a glass there is a subtle, but quite noticeable whiff of struck match on the nose and I personally find this to be pleasant. It quickly dissipates and I only perceive it for the first couple of sips. For those who don’t find this aspect enjoyable you could simply let the beer sit for a few minutes to permit the Sulfur Dioxide to outgas from the beer glass. Or drink the beer from the bottle since you won’t perceive the nose of the beer.

Higher alcohols (fusel oils)

Higher alcohols are alcohols which have more than 2 carbons in the chemical chain (ethanol has two carbon atoms). They are termed higher alcohols because they have a higher molecular weight. Excess amounts of higher alcohols in beer can be described as providing a harshness to the beer and reportedly they create hangovers. Higher gravity beers tend to have increased amounts of higher alcohols.

Purpose of the lagering phase

The two main processes that occur during lagering is precipitation and flavor maturation. These processes occur simultaneously but certain conditions may favor one process over the other.

Precipitation process

During lagering both yeast and proteins/polyphenols will precipitate (settle out). Precipitation of both increases with colder temperature.

Much of the yeast precipitates via flocculation during the primary fermentation phase but this also continues during lagering where the cold conditions encourage remaining yeast cells to drop out solution.

 Proteins within the beer carry a positive charge while polyphenols carry a negative charge. These two compounds are attracted to each other and from larger complexes: proteins/polyphenols. A law of physics called Stokes law states that the larger the diameter of the particle the faster it will settle to the bottom of the fermenter. Colder temperatures promote this activity.

Flavor maturation

This aspect of lagering is still a topic of continuing scientific study as discussed in the book “Brewing: Science and Practice” by Briggs et al (Chapter 15):

“Flavour improves during the maturation process but this flavor improvement is difficult to characterize and optimize.”


“Changes in the level of Sulphur compounds do occur in maturation but this is incompletely understood.”

During lagering the yeast are still active albeit at a slower level due to the cold temperatures. The colder the temperature the more slowly the yeast will process chemical compounds. If you fermented with a lager yeast strain which created a fair amount of sulfur compounds, and you do not want those sulfur compound flavors (e.g., rotten egg flavor) in the beer you need to provide conditions to permit the sulfur compounds to be processed. A warmer temperature (e.g., above freezing temperature) is best suited to promote yeast activity (i.e., biochemistry).

Considerations for lagering

Lagering vessel choices

In homebrewing the two main choices for lagering vessels are carboys or corny kegs. This was discussed in the companion article:

How to Make a Lager Beer

Commercial brewers use large stainless steel tanks to lager their beers. Some breweries use vertically oriented tanks while others choose to use horizontal tanks. The size of the brewery and the amount of floor space influence these decisions; horizontal tanks require more square feet of space while vertical tanks require high bay facilities. Some advantages of horizontal tanks are that there is less vertical space for the precipitation process (i.e., smaller distance for the yeast and protein/polyphenol aggregates to fall out of suspension) and the decreased hydrostatic pressure may be beneficial for the lager yeast strain selected.

Horizontal Lagering Tank

I have read that some breweries stack horizontal tanks which is an option if the brewery has the vertical space to do this.

During my tour of Únětický Pivovar in the Czech Republic, I saw their lagering tanks and they chose to lager in vertical tanks:

Vertical Lagering Tanks

Duration of the lagering phase

As previously discussed, the condition of the beer at the completion of primary fermentation will impact the need for further maturation during lagering. As a case in point, Dr. Charlie Bamforth who is now retired from his position as head of the UC Davis Brewing Program, has discussed in various forums that while he was employed by Bass Brewing they would conduct a stepped up temperature fermentation for Carling Lager and then conduct a very abbreviated lagering phase: just a few days at – 1 °C (-30.2 °). The concept being that all flavor aspects are addressed during the primary fermentation and no further flavor maturation was required during lagering. Perhaps the lager yeast strain used to ferment Carling produces little in the way of sulfur compounds?

In contrast I took tours of a number of lager breweries in the Czech Republic (Pilsner Urquell, Staropramen, and Únětický Pivovar) and they mentioned they lager for 30 days (1 month). I also listened to a podcast where the head brewer of Weihenstephaner (Tobias Zollo) stated they lager their beers for one month.

Original Gravity of the beer

Below is a repeat of a discussion in the article “How to Make Lager Beer”:

In the book “New Brewing Lager Beer” Greg Noonan suggested that lower gravity beers should be lagered for something like 3 - 7 days for each 2 °P of the OG of the beer. As one example, for a 1.040 (10 °P) beer a lagering duration of 15 – 35 days is needed. For mid-higher gravity beers, he suggests 7 – 12 days per 2 °P. For example, for a 1.048 (12 °P) beer 42 – 72 days of lagering would be suggested.  A notion that more time should be devoted for lagering higher OG beers is evident here.

I previously commented that when brewing higher gravity beers there is a tendency to produce higher alcohols. During the lagering phase the yeast can convert those higher alcohols to esters via a process referred to as esterification. An extended lagering period could be beneficial for a beer style such as a Doppelbock.

What temperature to use for lagering

How cold you decide to lager your beers will be dependent on a number of factors:

  • Is flavor maturation needed for the batch of beer?
  • Are you seeking a quick turnaround beer?
  • What level of clarification is desired?

I think it is safe to say that lagering should occur at cold temperatures (e.g., less than 40 °F) but how close to freezing (or below freezing for the case of Carling Lager) you get is a matter of choice. Lagering near freezing will optimize the precipitation process but perhaps at the risk of flavor maturation.

As previously discussed, for Carling Lager the brewery decides to just lager for a few days at a very cold temperature. I have never tasted Carling Lager which is an Adjunct Lager but from my readings this is not a beer that is thought of highly by non-mainstream beer drinkers. Perhaps there is a downside to lagering short and too cold?

Can you lager the beer too long?

There are some brewing industry folks who will state that lagering too long can be detrimental.

In the podcast discussion with Tobias Zollo, head brewer of Weihenstephaner, he mentioned he will not lager longer than 5 weeks since otherwise yeast autolysis introduces negative flavors to the beer. Maybe this is more of a concern at the scale of commercial brewing vs. the scale of homebrewing?

I can report that in my homebrewing practices I utilize a duration of 1 month for lagering at 38 °F for my moderate gravity lagers and this works for me. Each homebrewer should experiment and decide what mix of lager yeast strain, lagering temperature and lagering duration best suits their personal palate.

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